Hepatitis C virus (HCV) infection resolves spontaneously in humans and chimpanzees with strong, sustained cellular immune responses. During the last funding period we documented long-lived CD4+ and CD8+ memory T cells in immune chimpanzees that were required for rapid control of a second virus infection. Why T cell immunity fails during chronic infection is still not understood and is the major focus of this proposal. Mutational escape of most MHC class I epitopes was observed in viruses from persistently infected chimpanzees but this is probably not the only mechanism of CD8+ T cell evasion. Preliminary data indicate that some HCV-specific CD8+ T cells make the anti-inflammatory cytokine IL-10 instead of IFN-gamma during persistent infection. In Specific Aim 1 we will investigate the hypothesis that IL-10 inducing epitopes are not under immune selection pressure because this skewed cytokine response facilitates virus persistence. Specific Aim 2 proposes direct visualization of CD8+ T cells in the persistently infected liver with MHC class I tetramers to determine if ineffective virus control is associated with an immature phenotype and/or lack of effector molecules like perforin. In Specific Aim 3 we will use MHC class II tetramer technology to determine if CD4+ T cells are also compartmentalized to the liver but lack adequate helper activity for HCV clearance, perhaps because of mutations that alter epitope recognition. Finally, the antiviral function of CD4+ and CD8+ T cells in persistently infected animals will be tested directly by superinfection with wild-type HCV (Specific Aim 4). Our four specific aims are to: Specific Aim 1. Map MHC class I epitopes targeted by IL-10 or IFN-gamma producing CD8+ T cells and compare their susceptibility to immune selection pressure. Specific Aim 2 Determine if CD8+ T cells in liver have an immature phenotype that predicts a lack of in vivo effector function. Specific Aim 3. Define the repertoire and frequency of HCV-specific CD4+ T cells in the liver of individuals with persistent and resolved infections. Specific Aim 4. Superinfect persistently viremic chimpanzees with wild-type HCV to test directly the in situ responsiveness of CD4+ and CD8+ T cells.